Can 18-300mm replace 16-85 and 70-300 combination?

I think you are a little confused, as I understand it, the focal length marked on a DX or FX lens is the focal length of the lens! On a DX body the smaller sensor effectively crops the image and gives an "effective" focal length depending on the size of the sensor, e.g. a 35mm lens on a DX body has an effective field of view of 52mm.

On a FX body it is 35mm and an "effective" field of view of 35mm as the 35mm full frame sensor has been taken as the reference sensor size. Take a look at the focal length of P&S cameras you will see focal lengths of 4 - 6mm but their effective field of view will be around 28 / 35mm depending on the sensor size.

With regard to seeing different FOV when setting 2 lenses at the same focal length on the same body this is "focus breathing" where the focal length of the lens actually changes depending on focus distance. Complex zooms are prone to this effect. Hear is a quote from Thom Hogan from one of his lens tests:

Some lens designs "breath." That's the term for changing focal length with focus distance. The 18-200mm is a notorious heavy breather: it is so significantly short of 200mm at its closest focus distance that even casual users notice this. You can calculate the focal length of a lens at its closest focus distance by using the formula minimum_focus / ((1/reproduction_ratio) + reproduction_ratio + 2). The Nikon 17-55mm DX, for instance, works out to be 50mm under that formula, which is pretty much a non-breather. The older 70-200mm calculates to 182mm, which is still pretty much a non-breather (it's only lost 7% of its focal length). But the new 70-200mm? Well, it's a pretty heavy breather: it loses 29% of its focal length as you focus closer.
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Ian